Before the development of less invasive approaches, the principal mode of treatment for occluded arteries was bypass surgery and, in the case of occlusions in the coronary arteries, coronary artery bypass surgery. Coronary artery bypass surgery is a highly invasive procedure in which the chest cavity is opened to expose the heart to provide direct surgical access to the coronary arteries. The procedure also includes the surgical removal of blood vessels from other locations in the patient's body (e.g., the sapheneous vein) which then are grafted surgically to the coronary arteries to bypass the occlusions. The recuperative period is lengthy with considerable discomfort to the patient.
The use of less invasive, catheter-based, intravascular techniques has developed for several decades and may be considered as the preferred mode of treatment for those patients amenable to such treatment. Typically, the intravascular procedures, such as angioplasty, atherectomy, and stenting require preliminary navigation of a guidewire through the patient's arteries to and through the occlusion. This guidewire, so placed, serves as a rail along which catheters can be advanced directly to and withdrawn from the target site. Total occlusions often cannot be treated with such minimally invasive intravascular approaches because of the inability to advance a guidewire through the stenosis. Typically patients with such occlusions have been treatable, if at all, by pass surgery. Although in some instances, the physician may be able to force a guidewire through a total occlusion if the occluding material is relatively soft, that may present serious risks of perforating the artery. Arterial perforation can be life threatening.
The difficulties presented when trying to cross a total or near-total occlusion are compounded by the typical manner in which the anatomy of an occluded artery is diagnosed. Conventionally, such diagnosis involves an angiographic procedure in which a radiopaque contrast liquid is injected into the artery upstream of the occlusion and a radiographic image is made. The resulting image is that of the compromised lumen which necessarily differs from the natural arterial lumen. Although with angiographic visualization techniques, the physician can determine the location of the occluded region and an indication of the degree of obstruction, angiographic images do not provide a clear understanding of where, in the occluded region, the natural boundaries of the vessel are located.
As used herein, the term “severe occlusion” or “severe obstruction” is intended to include total occlusions as well as those occlusions and stenoses that are so restrictive as to require preliminary formation of a passage through the occlusion in order to receive additional intravascular therapeutic devices. Such occlusions may have various causes and may occur in the arterial or venous systems. Total or near total occlusions may occur as a consequence of the build-up of plaque or thrombus, the latter being problematic in arteries as well as in the venous system. For example, deep veined thrombus and thrombotic occlusion of vein grafts are serious conditions requiring treatment.
More recently, techniques and systems have been developed to visualize the anatomy of vascular occlusions by using intravascular ultrasound (IVUS) imaging. IVUS techniques are catheter-based and provide a real-time sectional image of the arterial lumen and the arterial wall. An IVUS catheter includes one or more ultrasound transducers at the tip of the catheter by which images containing cross-sectional information of the artery under investigation can be determined. IVUS imaging permits visualization of the configuration of the obstructing material and, in varying degrees, the boundaries of the intimal and medial layers of the arterial wall. One common type of IVUS imaging catheter system typically includes an arrangement in which a single transducer at the distal end of the catheter is rotated at high speed (up to about 2000 rpm) to generate a rapid series of 360-degree ultrasound sweeps. Such speeds result in generation of up to about thirty images per second, effectively presenting a real-time sectional image of the diseased artery. The transducer is mounted on the end of a drive shaft that is connected to a motor drive at the proximal end of the catheter. The rotating transducer is housed in a sheath that does not interfere with the ultrasound and protects the artery from the rapidly spinning drive shaft. Thus, an IVUS imaging catheter may be advanced to the region of an occlusion using conventional angiographic techniques and then may be operated to provide real-time sectional images of the vascular lumen in the arterial wall, including the occluding material and intimal and medial layers of the artery wall.
Proposals and development efforts have been made to combine IVUS imaging techniques with a catheter adapted to remove obstructive material from the artery. One such arrangement has been to provide a catheter having spark erosion electrodes by which obstructive plaque can be ablated in conjunction with an IVUS imaging system by which the anatomy of the artery and obstruction may be visualized. The objective of such catheters is to provide the physician with information as to the location and the characteristics of the stenosis, coupled with the ability to provide a controlled spark erosion of the occlusive materials. Ideally, the system should remove only plaque deposited on the inner luminal surface of the artery and in the innermost intimal layer which, typically, will have been thickened, often irregularly, as a consequence of the plaque deposits. Typically, there is little development of plaque within the medial layers of the artery. The system desirably should have the ability to remove plaque-laden intima without causing dissections, releasing obstructive material into the bloodstream or provoking other major adverse side effects.
Such systems are described, for example, in Slager, et al., Directional Plaque Ablation by Spark Erosion Under Ultrasound Guidance: First Evaluation of a Catheter Incorporating Both Techniques, Dissertation Public Presentation Dec. 17, 1997, and included as Chapter 8 in Slager, Cornelis Jacob, “Removal of Cardiovascular Obstructions by Spark Erosion”, ISBN 90-9011073-9, printed by ICG Printing Dordrecht. The system is described as including a catheter adapted to contain a stainless steel tubular rotatable drive shaft. A tip, mounted at the distal end of the drive shaft, includes a circular ultrasound transducer and a sparking electrode. Wires for the ultrasound signals and a high-voltage wire to transmit RF energy to the active electrode extend through the lumen in the drive shaft. A slip ring construction near the proximal end of the drive shaft provides electrical connection between the electrode wire and the spark erosion generator. The proximal end of the drive shaft is connected to a motor drive unit. The drive shaft is selected to be torsionally stiff to synchronize the angular orientation of the tip and the motor drive unit. The device is operated at a tip rotation frequency of 12.5 Hz (750 rpm). Timing of the start of the spark erosion pulse is described as related to the timing signals obtained from the ultrasound imaging equipment. Another such system is described in U.S. Pat. No. 6,394,956 which also describes a catheter having a drive shaft that rotates an ultrasound transducer and electrode at a rate of approximately 1500 to 2000 rpm. These RF ablation systems using IVUS imaging are intended to image, in real time, at about twelve to about thirty image frames per second. They tend to be very expensive systems due to the complexity of the real time electronics and related mechanical features.
It would be desirable to provide a low cost, simplified, intravascular ultrasound system and to provide such a system to guide a therapeutic device for the treatment for total vascular occlusions and it is among the general objects of the present invention to provide such systems and techniques.